An MI cable is a cable in which one or a plurality of wires are held in a metal sheath with a mineral insulating powder such as magnesia, silica, alumina, or the like, interposed therebetween.
An MI cable is used, for example, for a sheathed thermocouple, a sheathed resistance thermometer, a microheater, or the like. As is well known, one or a plurality of pairs of thermocouple wires are held as the wires of an MI cable used for a sheathed thermocouple, typically three or four copper wires are held as the wires of an MI cable used for a sheathed resistance thermometer, and one or two electrical resistance wires are held as the wires of an MI cable used for a microheater.
An MI cable is used mainly in a high temperature environment or corrosive environment that is intolerable for an ordinary soft cable having an insulating material or a covering material made of polyethylene, vinyl, rubber, or the like. At an end of the MI cable, the wires of the MI cable are connected to lead wires of a soft cable. For example, the thermocouple wires of an MI cable for a sheathed thermocouple are connected to lead wires of a soft cable configured by thermocouple compensating lead wires, the copper wires of an MI cable for a sheathed resistance thermometer are connected to lead wires of a soft cable configured by copper lead wires, and the electrical resistance wires of an MI cable for a microheater are connected to lead wires of a soft cable for supplying a current to these wires.
The structure for connecting the wires at an end of the MI cable to the lead wires of the soft cable is required to realize a function of insulating the interior of the MI cable from the outside air by sealing in order to prevent the insulating properties of the mineral insulating powder inside the MI cable from being lowered by the entrance of moisture, and a function of preventing breakage of a thin wire exposed from the metal sheath of an MI cable especially in the case of an MI cable having a small outer diameter.
Patent Documents 1 and 2 describe a simple structure for an end of an MI cable, in which a portion connecting wires exposed from the metal sheath to lead wires of a soft cable is covered by a metal sleeve, and the metal sleeve is filled with an insulating material.
Hereinafter, a conventional structure for an MI cable having two wires will be described. Note that the same can be applied to those having a different number of wires.
FIG. 7 is a typical example of a simple structure for an end of an MI cable conventionally used for a sheathed thermocouple, corresponding to the structure shown in FIG. 2 of Patent Document 1. FIG. 7 is a cross-sectional view, wherein a soft cable 5 is shown as a view from the outside.
A portion connecting ends of two wires 3 exposed from an MI cable 1, in which the wires 3 are held in a cylindrical metal sheath 2 with a mineral insulating powder 4 interposed therebetween, to ends of lead wires 6 exposed by stripping off a cover 7 of the soft cable 5 is held in a metal sleeve 10, and the metal sleeve 10 is filled with a resin 20. The interior of the MI cable 1 is insulated from the outside air by the resin 20, so that the insulating properties are prevented from being lowered by the entrance of moisture, and breakage of the thin wires 3 exposed from the MI cable 1 is prevented by the metal sleeve 10 and the resin 20.
Although not described in Patent Document 1, the interior of the MI cable 1 is often sealed from the outside, by removing in advance the mineral insulating powder at an end of the MI cable 1 and filling the space formed by the removal with a resin sealing member 14, in order to reliably insulate the interior of the MI cable 1 from the outside air. Furthermore, an end 15 of the metal sleeve 10 on the MI cable-side is often welded or brazed around the entire circumference of the metal sheath 2 of the MI cable 1.
FIG. 8 is a typical example of a simple structure for an end of an MI cable conventionally used for a microheater, corresponding to the structure shown in FIG. 1(a) of Patent Document 2. FIG. 8 is a cross-sectional view, wherein the soft cable 5 is shown as a view from the outside.
A portion connecting ends of two wires 3 exposed from the MI cable 1, in which the wires 3 are held in a cylindrical metal sheath 2 with a mineral insulating powder 4 interposed therebetween, to ends of lead wires 6 exposed by stripping off a cover 7 of the soft cable 5 is held in a metal sleeve 10, as in the example shown in FIG. 7.
In the case of a microheater, the wires 3 of the MI cable 1 are electrical resistance wires, and the temperature at the end of the MI cable 1 may increase due to heat generated by the wires 3. Thus, in view of heat resistance, a resin is seldom used therein. Instead, a mineral insulating powder 21 is often used to fill the metal sleeve 10, and a heat-resistant sealing member 14 such as glass or ceramic is often used to seal the end of the metal sleeve 10. Furthermore, since the mineral insulating powder 21 has no sealing properties, an end 15 of the metal sleeve 10 on the MI cable-side is always welded or brazed around the entire circumference of the metal sheath 2 of the MI cable 1. The interior of the MI cable 1 is insulated from the outside air by the welding or the brazing around the entire circumference of the sealing member 14, and breakage of the wires 3 exposed from the MI cable 1 is prevented by the metal sleeve 10 and the mineral insulating powder 21.
Note that, in FIGS. 7 and 8, the same constituent components as those in embodiments of the present invention are denoted by the same reference numerals as those in drawings illustrating the embodiments of the invention. The same can be applied to FIG. 9 described later.
Other examples of a simple structure for an end of an MI cable used for a sheathed thermocouple include a structure as shown in FIG. 2 of Patent Document 3 in which a metal holding arm that bridges an MI cable and a soft cable is provided as a mechanical reinforcing member instead of a metal sleeve, and the entire connecting portion including the holding arm is molded with a resin. Also in this case, the end of the MI cable 1 is often sealed by a resin in advance, in order to reliably insulate the interior of the MI cable from the outside air.